Seam welding process and seam welding apparatus

ABSTRACT

Seam welding process and apparatus, wherein two plate-like workpieces are fed in a feeding direction toward a pair of roller electrodes, by guide rollers rotated by a driving device, while the workpieces are guided by a guiding device such that the edge portions overlap each other with a predetermined lap amount so that the overlapping edge portions are squeezed by the roller electrodes, and the edge portions are continuously welded together with a welding current being applied between the roller electrodes.

FIELD OF THE ART

This invention relates to a seam welding process and a seam weldingapparatus for continuously welding together two plate-like workpieces byoverlapping the workpieces at their edges and applying a welding currentbetween a pair of roller electrodes while the overlapping edge portionsof the two plate-like workpieces are squeezed, or squeezed to be mashed,by and between the pair of roller electrodes.

BACKGROUND TECHNOLOGY

High-speed welding is possible by seam welding in which two plate-likeworkpieces are continuously welded together at their overlapping edgeportions while the edge portions are squeezed or squeezed to be mashedby and between a pair of roller electrodes, with a welding current beingapplied between the pair of roller electrodes. Since the seam welding issuitable for mass production, it is widely employed in production linesfor steel plates, automobiles, etc.

In such seam welding, a force of the pair of roller electrodes appliedto the overlapping edge portions of the two plate-like workpiecesgenerates an expanding force at the edge portions. To prevent theoverlapping edge portions of the two plate-like workpieces from movingaway from each other, it is required to firmly hold the plate-likeworkpieces together. This requirement is particularly high, in a mashseam welding operation in which the two plate-like workpieces arecontinuously welded while the overlapping edge portions are squeezed soas to be mashed by the pair of roller electrodes, with a large force ofexpansion generated at the mashed edge portions in the direction awayfrom each other.

Conventionally, therefore, two plate-like workpieces 12_(H), 12_(M) arefirmed held by upper and lower clamping members 120 of a clampingmechanism 122, as shown in FIG. 10. The two plate-like workpieces12_(H), 12_(M) are continuously welded together at their overlappingedge portions while the upper and lower clamping members 120 are movedrelative to a pair of roller electrodes 20_(U), 20_(S). Examples of sucha conventional seam welding apparatus are disclosed in JP-A-59-223179and JP-A-61-52994.

In the conventional seam welding apparatus described above, however, theupper and lower clamping members 120 or the pair of roller electrodes20_(U), 20_(S) must be separated from the welded plate-like workpieces12_(H), 12_(M) and moved to the original position after the seam weldingprocess is completed by relative movement of the clamping mechanism 122and the roller electrodes 20_(U), 20_(S). Therefore, the seam weldingapparatus is undesirably complicated or large-sized due to a feedingmechanism for effecting the relative movement of the clamping mechanismand electrodes. Where the roller electrodes 20_(U), 20_(S) are adaptedto be returned to the original position, there is a drawback that theproduction efficiency of the seam welding apparatus is lowered due tointerruption of adjustment of the electrodes.

The present invention was made in the light of the above background. Itis an object of the invention to provide a seam welding process and aseam welding apparatus, which permit continuous welding without relativemovement of a clamping mechanism and roller electrodes, and which assurehigh production efficiency with small-sized equipment.

DISCLOSE OF THE INVENTION

The above object may be achieved according to the principle of thepresent invention, which provides a seam welding process of continuouslywelding two plate-like workpieces at overlapping edge portions thereofwhile the overlapping edge portions are squeezed by a pair of rollerelectrodes disposed rotatably about respective two parallel axesthereof, comprising: (a) a moving step of feeding the two plate-likeworkpieces in one direction toward the roller electrodes, by using aplurality of pairs of guide rollers to pinch the two plate-likeworkpieces in a direction of thickness thereof; (b) a guiding step ofguiding the two plate-like workpieces being fed in the one direction inthe moving step, such that the edge portions overlap each other with apredetermined lap amount so that the overlapping edge portions aresqueezed by the pair of roller electrodes; and (c) a welding step ofcontinuously welding the edge portions of the two plate-like workpiecessqueezed by the pair of roller electrodes, by applying a welding currentbetween the pair of roller electrodes.

In the present process, the two plate-like workpieces pinched by theplurality of pairs of guide rollers are fed by rotation of the guiderollers in the direction perpendicular to the axes of theabove-indicated roller electrodes. In this feeding step, the guidingstep is performed to guide the two plate-like workpieces so that theedge portions overlapping each other with the predetermined lap amountare squeezed by the above-indicated pair of roller electrodes. In thewelding step, the edge portions of the two plate-like workpiecessqueezed by the above-indicated pair of roller electrodes arecontinuously welded with the welding current being applied between thepair of roller electrodes. According to the process of the presentinvention, therefore, a seal welding apparatus is less complicated andsmaller in size than an apparatus which uses a clamping mechanism formoving the plate-like workpieces to the pair of roller electrodes whilethe workpieces are held by upper and lower clamping members. The presentapparatus does not require a mechanism as provided in the apparatus withthe above clamping mechanism, for returning the upper and lower clampingmembers after its removal from the welded plate-like workpieces.Further, the present process is free from deterioration of seam weldingefficiency due to movements of the roller electrodes, and permits higherseam welding efficiency than in an apparatus adapted to reciprocate theroller electrodes relative to the upper and lower clamping members.

Preferably, the guiding step comprises providing a guide member which isfixedly disposed upstream of the pair of roller electrodes and which hasa pair of guide surfaces parallel to a direction perpendicular to theaxes of the roller electrodes, and guiding the two plate-like workpiecessuch that end faces of the overlapping edge portions of the twoplate-like workpieces are in sliding contact with the pair of guidesurfaces. In this case, it is possible to improve the positioningaccuracy of the overlapping edge portions of the two plate-likeworkpieces, with a simple arrangement.

It is also preferable that the guiding step comprises providing at leasttwo pairs of guide rollers which are located on opposite sides of theguide member to pinch the two plate-like workpieces in the direction ofthickness and which generate forces including components acting in adirections toward the guide surfaces of the guide member while the twoplate-like workpieces are fed, whereby the end faces of the overlappingedge portions of the two plate-like workpieces are brought into slidingcontact with the guide surfaces. In this case, the above-indicated twoplate-like workpieces are forced onto the guide surfaces of the guidemember while the plate-like workpieces are fed in the feeding directionby the at least two pairs of guide rollers. This arrangement isadvantageous for a reduced number of sliding portions than in anarrangement wherein guide plates are provided for forcing the plate-likeworkpieces onto the guide surfaces of the guide member.

It is also preferable that the welding process comprises welding the twoplate-like workpieces while the edge portions are squeezed to be mashedby a pair of roller electrodes which are disposed rotatably aboutrespective two parallel axes and which are biased toward each other.This arrangement does not require a chamfering operation or otherpreliminary treatment of the edge portions of the plate-like workpieces.In this so-called "mash seam welding", a large clamping mechanism isconventionally required for resisting a large expanding force generatedby mashing of the two plate-like workpieces by the pair of rollerelectrodes. In this respect, the present process is advantageous overthe conventional process.

It is also preferable that the present seam welding process furthercomprises a step of squeezing, in the welding step, the two plate-likeworkpieces in the direction of thickness with a predetermined squeezingforce by two pairs of retainer rollers disposed on opposite sides of thepair of roller electrodes, respectively, such that the two pairs ofretainer rollers generate friction forces resisting expanding forces ofthe two plate-like workpieces in the welding step, to thereby preventmovements of the two plate-like workpieces away from each other. In thiscase, the movements of the edge portions of the plate-like workpiecesaway from each other in the welding step are prevented, whereby theaccuracy of the welding position is effectively improved.

It is also preferable that the present seam welding process furthercomprises a pressing step of pressing portions of the plate-likeworkpieces which are located between the roller electrodes and theretainer rollers in an axial direction of the roller electrodes and atwhich convex deformation is expected to take place. This arrangementprevents the movements of the edge portions of the plate-like workpiecesaway from each other due to the convex deformation of the plate-likeworkpieces, and is therefore effective to prevent the resultingdifficulty in forming the weld nugget within the two plate-likeworkpieces, and the consequent reduction in the weld strength.

It is also preferable that the present seam welding process furthercomprises an electrode driving step of rotating the roller electrodes soas to transfer a driving force to the plate-like workpieces in thefeeding direction. In this case, the driving force for feeding theplate-like workpieces is transferred from the roller electrodes to theplate-like workpieces, so as to restrict the expanding force of the pairof plate-like workpieces, which would cause movements of the edgeportions of the plate-like workpieces away from each other. Thus, thepresent arrangement is effective to prevent the difficulty in formingthe weld nugget within the two plate-like workpieces.

It is also preferable that the present seam welding process furthercomprises a coolant supply step of supply a coolant fluid to portions ofthe plate-like workpieces which are squeezed by the roller electrodes,for cooling portions of the plate-like workpieces which are heated byseam welding by the roller electrodes. In this instance, the supply ofthe coolant fluid is effective to reduce the expanding force of the pairof plate-like workpieces which is generated due to heat at the weldnugget formed within the plate-like workpieces between the rollerelectrodes upon application of the electric current between the rollerelectrodes. Thus, the present arrangement is effective to prevent thedifficulty in forming the weld nugget within the two plate-likeworkpieces between the roller electrodes.

The above process of the present invention may be suitably practiced bya seam welding apparatus adapted to continuously weld two plate-likeworkpieces at overlapping edge portions thereof while the overlappingedge portions are squeezed by a pair of roller electrodes disposedrotatably about respective two parallel axes thereof and while a weldingcurrent is applied between the pair of roller electrodes, the apparatuscomprising: (a) a pair of roller electrodes disposed rotatably aboutrespective two parallel axes thereof and biased toward each other; (b) aplurality of pairs of guide rollers which are disposed on opposite sidesof the two plate-like workpieces in a direction of thickness of theworkpieces, rotatably about axes thereof substantially parallel to theaxes of the roller electrodes; (c) a driving device for rotating atleast a portion of the plurality of pairs of guide rollers in apredetermined rotating direction, to feed the two plate-like workpiecesin one direction while the two plate-like workpieces are pinched by theguide rollers; and (d) a guiding device for guiding the two plate-likeworkpieces being fed in the one direction by the driving device, suchthat the edge portions overlap each other with a predetermined lapamount so that the overlapping edge portions are squeezed by the pair ofroller electrodes.

In the present apparatus, the two plate-like workpieces pinched by theplurality of pairs of guide rollers are fed by rotation of the guiderollers by the driving device in the direction perpendicular to the axesof the above-indicated roller electrodes. During the feeding of the twoplate-like workpieces, these workpieces are guided by the guiding deviceso that the edge portions overlapping each other with the predeterminedlap amount are squeezed by the above-indicated pair of rollerelectrodes. According to this arrangement, the edge portions of the twoplate-like workpieces squeezed by the above-indicated pair of rollerelectrodes are continuously welded with the welding current beingapplied between the pair of roller electrodes. Therefore, the presentseam welding apparatus is less complicated and smaller in size than anapparatus which uses a clamping mechanism for moving the plate-likeworkpieces to the pair of roller electrodes while the workpieces areheld by upper and lower clamping members. The present apparatus does notrequire a mechanism as provided in the apparatus with the above clampingmechanism, for returning the upper and lower clamping members after itsremoval from the welded plate-like workpieces. Further, the presentapparatus is free from deterioration of seam welding efficiency due tomovements of the roller electrodes, and permits higher seam weldingefficiency than in an apparatus adapted to reciprocate the rollerelectrodes relative to the upper and lower clamping members.

Preferably, the guide device includes a guide member which is fixedlydisposed upstream of the pair of roller electrodes and which has a pairof guide surfaces parallel to a direction perpendicular to the axes ofthe roller electrodes, the guide device positioning the two plate-likeworkpieces relative to each other during feeding of the plate-likeworkpieces such that end faces of the overlapping edge portions of thetwo plate-like workpieces are in sliding contact with the pair of guidesurfaces. In this case, it is possible to improve the positioningaccuracy of the overlapping edge portions of the two plate-likeworkpieces, with a simple arrangement.

It is also preferable that the guide device has at least two pairs ofguide rollers located on opposite sides of the guide member in adirection of thickness thereof to pinch the two plate-like workpieces,the at least two pairs of guide rollers guiding the two plate-likeworkpieces such that the two plate-like workpieces approach the guidingsurfaces of the guide member as the two plate-like workpieces approachthe roller electrodes, whereby the end faces of the overlapping edgeportions of the two plate-like workpieces are brought into slidingcontact with the guide surfaces. In this case, the above-indicated twoplate-like workpieces are forced onto the guide surfaces of the guidemember while the plate-like workpieces are fed in the feeding directionby the at least two pairs of guide rollers. This arrangement isadvantageous for a reduced number of sliding portions than in anarrangement wherein guide plates are provided for forcing the plate-likeworkpieces onto the guide surfaces of the guide member.

It is also preferable that the pair of roller electrodes are adapted toweld the edge portions of the plate-like workpieces while the edgeportions of the two plate-like workpieces are squeezed to be mashed bythe pair of roller electrodes. This arrangement does not require achamfering operation or other preliminary treatment of the edge portionsof the plate-like workpieces. In this so-called "mash seam welding", alarge clamping mechanism is conventionally required for resisting alarge expanding force generated by mashing of the two plate-likeworkpieces by the pair of roller electrodes. In this respect, thepresent process is advantageous over the conventional process.

It is also preferable that the seam welding apparatus further comprises:two pairs of retainer rollers which are disposed on opposite sides ofthe pair of roller electrodes, rotatably about a pair of axes parallelto the axes of the pair of roller electrodes, the two pairs of retainerrollers squeezing the two plate-like workpieces in the direction ofthickness while the two plate-like workpieces are squeezed by the pairof roller electrodes; and a squeezing force applying device for applyingto the two pairs of retainer rollers squeezing forces for generatingfriction forces enough to resist expanding forces of the two plate-likeworkpieces which are generated during welding thereof by the pair ofroller electrodes. In this case, the movements of the edge portions ofthe plate-like workpieces away from each other in the welding step areprevented, whereby the accuracy of the welding position is effectivelyimproved.

It is also preferable that the seam welding apparatus further comprisesa pressing device for pressing portions of the plate-like workpieceswhich are located between the roller electrodes and the retainer rollersin an axial direction of the roller electrodes and at which convexdeformation is expected to take place. This arrangement prevents themovements of the edge portions of the plate-like workpieces away fromeach other due to the convex deformation of the plate-like workpieces,and is therefore effective to prevent the resulting difficulty informing the weld nugget within the two plate-like workpieces.

It is also preferable that the presser device includes auxiliary rollerswhich are disposed between the roller electrodes and the retainerrollers in the axial direction of the roller electrodes, rotatably aboutaxes parallel to the axes of the roller electrodes, for pressing theplate-like workpieces while permitting the plate-like workpieces to befed in the feeding direction, or alternatively auxiliary skids which aredisposed between the roller electrodes and the retainer rollers in theaxial direction of the roller electrodes, for sliding contact with theplate-like workpieces.

It is also preferable that the seam welding apparatus further comprisesan electrode driving device for rotating the roller electrodes so as totransfer a driving force to the plate-like workpieces in the feedingdirection. In this case, the driving force for feeding the plate-likeworkpieces is transferred from the roller electrodes to the plate-likeworkpieces, so as to restrict the expanding force of the pair ofplate-like workpieces, which would cause movements of the edge portionsof the plate-like workpieces away from each other. Thus, the presentarrangement is effective to prevent the difficulty in forming the weldnugget within the two plate-like workpieces.

It is also preferable that the seam welding apparatus further comprisesa coolant supply device for supplying a coolant fluid to portions of theplate-like workpieces which are squeezed by the roller electrodes, forcooling portions of the plate-like workpieces which are heated by seamwelding by the roller electrodes. In this instance, the supply of thecoolant fluid is effective to reduce the expanding force of the pair ofplate-like workpieces which is generated due to heat at the weld nuggetformed within the plate-like workpieces between the roller electrodesupon application of the electric current between the roller electrodes.Thus, the present arrangement is effective to prevent the difficulty informing the weld nugget within the two plate-like workpieces between theroller electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing a manner of feedingplate-like workpieces to explain a seam welding process of the presentinvention;

FIG. 2 is a plan view explaining an arrangement of guide rollers andretainer rollers used for feeding the plate-like workpieces, to explainthe seam welding process according to the embodiment of FIG. 1;

FIG. 3 is a view showing portions of guide members by which theplate-like workpieces are slidably guided, to explain a workpieceguiding step in the embodiment of FIG. 1;

FIG. 4 is a view explaining operations of the guide rollers and retainerrollers used in workpiece guiding and holding steps in the embodiment ofFIG. 1;

FIG. 5 is a view explaining a relationship between an expanding forcegenerated in a welding step and a distance from roller electrodes, inthe embodiment of FIG. 1;

FIG. 6 is a view explaining a relationship between the expanding forcegenerated in the welding step and the thickness of the plate-likeworkpieces squeezed by the roller electrodes;

FIG. 7 is a view in cross section showing the guide rollers and theirvicinity of a seam welding apparatus suitable for practicing the seamwelding process of FIG. 1, taken in a plane perpendicular to a feedingdirection of the plate-like workpieces;

FIG. 8 is a view showing in enlargement a portion of the seam weldingapparatus of FIG. 7;

FIG. 9 is a partly cut-away view explaining the roller electrodes andtheir vicinity of the seam welding apparatus of FIG. 7;

FIG. 10 is a perspective view explaining steps of a conventional seamwelding process using a clamping mechanism for clamping together a pairof plate-like workpieces;

FIG. 11 is a view explaining convex deformation F of the plate-likeworkpieces which may occur in a seam welding process;

FIG. 12 is a view explaining a relationship between the convexdeformation F shown in FIG. 11 and a distance between the rollerelectrodes and pressure rollers;

FIG. 13 is a view explaining a state of seam welding where the convexdeformation F shown in FIG. 11 does not occur;

FIG. 14 is a view explaining a state of seam welding where the convexdeformation F shown in FIG. 11 occurs;

FIG. 15 is a view explaining electrode rollers and retainer rollers andtheir vicinity in another embodiment of this invention;

FIG. 16 is a view explaining the arrangement of auxiliary rollers in theembodiment of FIG. 15;

FIG. 17 is a view explaining an operation of the auxiliary rollers toprevent convex deformation F in the embodiment of FIG. 15;

FIG. 18 is a view corresponding to that of FIG. 15, explaining theroller electrodes and retainer rollers and their vicinity in anotherembodiment of this invention;

FIG. 19 is a side view showing auxiliary skids in the embodiment of FIG.18;

FIG. 20 is a view explaining a roller electrode in another embodiment ofthis invention;

FIG. 21 is view explaining an operation of the roller electrode in theembodiment of FIG. 20;

FIG. 22 is a view explaining the operation of the roller electrode inthe embodiment of FIG. 20;

FIG. 23 is a view explaining an arrangement of a roller electrode andits vicinity in another embodiment of this invention; and

FIG. 24 is a perspective view showing a schematic arrangement of a seamwelding apparatus in another embodiment of this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

There will be described embodiments of this invention in detailreferring to the drawings. FIG. 1 through FIG. 6 are views explaining aseam welding process to be practiced according to this invention, whileFIG. 5 through FIG. 9 are views explaining a seam welding apparatus 10adapted to practice the seam welding process.

In FIGS. 1 and 2, there are shown a pair of left and right plate-likeworkpieces 12_(H), 12_(M) which are separate plates such as zinc steelplates having given shapes. These plate-like workpieces 12_(H), 12_(M)are fed in a predetermined feeding direction B by a plurality of sets ofguide rollers 14 (Feeding Step), while the two plate-like workpieces12_(H), 12_(M) are guided and positioned relative to each other by apair of upper and lower plate-like guide members 18_(U), 18_(S) insliding contact with guide surfaces 24_(U), 24_(S) such that edgeportions of the plate-like workpieces 12_(H), 12_(M) overlap each otherwith a predetermined lap amount (lap width) S (Guiding Step). The thuspositioned two plate-like workpieces 12_(H), 12_(M) are squeezed underpressure at their overlapping edge portions, by a pair of upper andlower roller electrodes 20_(U), 20_(S). In this condition, an electriccurrent supplied from a power source is applied between the pair ofroller electrodes 20_(U), 20_(S), whereby the two plate-like workpieces12_(H), 12_(M) are welded together at their overlapping edge portions,along a welding centerline A which is parallel to the feeding directionB and which passes a center of the lamp width S of the overlapping edgeportions (Welding Step). A set of retainer rollers 16 are disposed onopposite sides of the roller electrodes 20_(U), 20_(S), for holding thepair of plate-like workpieces 12_(H), 12_(M) under welding, with apredetermined squeezing force in the direction of thickness of theworkpieces, so as to prevent movement of the workpieces away from eachother (Holding Step).

Each of the above-indicated sets of guide rollers 14 consists of a pairof upper and lower guide rollers 14_(HU), 14_(HS) disposed to pinch theplate-like workpiece 12_(H) with a predetermined pinching force in thedirection of thickness of the workpiece, and another pair of upper andlower guide rollers 14_(MU), 14_(MS) disposed to pinch the otherplate-like workpiece 12_(M) with a predetermined pinching force in thedirection of thickness of the workpieces. The sets of guide rollers 14are arranged in the feeding direction B, and include at least one set ofguide rollers 14 which are positively driven. Each guide roller 14_(HU),14_(HS), 14_(MU), 14_(MS) of each set is supported rotatably about ahorizontal axis C1 which lies in a plane inclined a suitable angle θwith respect to a plane perpendicular to the welding centerline A andwhich is located above or below the welding centerline A. Each pair ofupper and lower guide rollers 14_(HU) and 14_(HS), or 14_(MU) and14_(MS) is given a suitable pinching force by a cylinder device 56_(H),56_(M) described below, for example, such that the pinching force actson the upper and lower guide rollers in a direction toward each other.The pinching force is determined so as to generate a friction forceenough to permit the upper and lower rollers to generate a force F_(R)consisting of a component F_(R2) for feeding the plate-like workpiece12_(H), 12_(M) in the feeding direction B and a component F_(R1) formoving the workpiece in the direction toward the guide members 18_(U),18_(S) as described below by reference to FIG. 4.

The above-indicated set of retainer rollers 16 consists of a pair ofupper and lower retainer rollers 16_(HU), 16_(HS) which are disposed onone side of the roller electrodes 20_(U), 20_(S) to hold the plate-likeworkpiece 12_(H) with a predetermined squeezing force in the directionof thickness of the workpiece, and another pair of upper and lowerretainer rollers 16_(MU), 16_(MS) disposed on the other side of theroller electrodes 20_(U), 20_(S) to hold the other plate-like workpieces12_(M) with a predetermined squeezing force in the direction ofthickness of the workpiece. Thus, the two pairs of retainer rollers 16are disposed such that the roller electrodes 20_(U), 20_(S) areinterposed between the retainer rollers 16. Each pair of upper and lowerretainer rollers 16_(HU) and 16_(HS), or 16_(MU) and 16_(MS) is given acomparatively large squeezing force by a cylinder device similar to thecylinder device 56_(H), 56_(M) described below, for example, such thatthe squeezing force acts on the upper and lower retainer rollers in adirection toward each other. The squeezing force is determined so as toprevent the edge portions of the pair of plate-like workpieces 12_(H),12_(M) under welding from moving away from each other. Each retainerroller 16_(HU), 16_(HS), 16_(MU), 16_(MS) is supported rotatably about ahorizontal axis C2 which lies in a plane perpendicular to the weldingcenterline A and which is located above or below the welding centerlineA. The plate-like workpieces 12_(H), 12_(M) under feeding, namely, underwelding are retained by the above-indicated squeezing force to preventthe movements of their edge portions away from each other (HoldingStep).

Each of the above-indicated pair of roller electrodes 20_(U), 20_(S) issupported rotatably about a horizontal axis C3 which lies in a planeperpendicular to the welding centerline A and which is located above orbelow the welding centerline A. The pair of roller electrodes 20_(U),20_(S) is given a comparatively large squeezing force by a cylinderdevice by a cylinder device 80 described below, for example, such thatthe squeezing force acts on the roller electrodes in a direction towardeach other. The squeezing force is determined so as to permit theoverlapping edge portions of the plate-like workpieces 12_(H), 12_(M) tobe mashed by an amount enough to reduce the overall lap thickness to theapproximate thickness of one of the workpieces 12_(H), 12_(M). Thesqueezing portion of the roller electrodes 20_(U), 20_(S) is indicatedat P in FIG. 2.

As shown in detail in FIG. 3, the above-indicated plate-like guidemembers 18_(U), 18_(S) are disposed with a predetermined amount of gapleft between mutually opposed end faces thereof, so as to extend alongthe welding centerline A and between the pair of upper and lower guiderollers 14_(HU), 14_(HS) for holding the one plate-like workpiece14_(GH), and the pair of upper and lower guide rollers 14_(MU), 14_(MS)for holding the other plate-like workpiece 12_(M), such that one oflongitudinal ends of the guide members 18U, 18S is spaced apart by apredetermined small distance from the pair of roller electrodes 20_(U),20_(S) in the longitudinal direction. The upper guide member 18_(U) hasa projecting rail 26_(U) formed at a portion of its end face on the sideof the plate-like workpiece 12_(M). The projecting rail 26_(U) has aguide surface 24_(U) for sliding contact with the end face of theplate-like workpiece 12_(H), The lower guide member 18_(S) has aprojecting rail 26_(S) formed at a portion of its end face on the sideof the plate-like workpiece 12_(H). The projecting rail 26_(S) has aguide surface 24_(S) for sliding contact with the end face of theplate-like workpiece 12_(M).

As shown in detail in FIG. 4, the rotation axis C1 of at least one set(all sets in this embodiment) of the guide rollers 14 disposed on theleft and right sides of the guide members 18_(U), 18_(S) is inclined apredetermined small angle θ (1°-3° in this embodiment) with respect tothe line perpendicular to the welding centerline A, so that theplate-like workpieces 12_(H), 12_(M) are driven or guided by the guideroller 14 in a direction (in the direction of the driving force F_(R)described below) which is inclined inwardly by the predetermined angle θwith respect to the feeding direction B.

When the guide roller 14_(MU) is rotated while the plate-like workpiece12_(M) is in abutting contact with the guide surface 24_(S), forexample, the driving force F_(R) produced by the guide roller 14_(MU)consists of a component F_(R1) which acts in the direction perpendicularto the feeding direction B, toward the welding centerline A or guidesurface 24_(S), and a component F_(R2) which acts in the directionparallel to the feeding direction B. Accordingly, the plate-likeworkpiece 12_(M) is fed while it is forced against the guide surface24_(S). Similarly, the plate-like workpiece 12_(H) is fed while it isforced against the guide surface 24_(U). Thus, the pair of plate-likeworkpieces 12_(H), 12_(M) are positioned relative to each other withtheir edge portions overlapping over the predetermined lap width S,before the workpieces are squeezed by the upper and lower rollerelectrodes 20_(U), 20_(S). In this arrangement, the pair of plate-likeworkpieces 12_(H), 12_(M) are automatically positioned relative to eachother during feeding thereof through a feeding path, even if theworkpieces do not have the predetermined relative position when they areintroduced into the feeding path.

When the overlapping edge portions of the two plate-like workpieces12_(H), 12_(M) are squeezed or mashed by the pair of upper and lowerroller electrodes 20_(U), 20_(S), there is generated an expanding forcewhich causes the overlapping edge portions to be moved in a directionaway from each other. This expanding force increases in the feedingdirection B from a position upstream of the roller electrodes 20_(U),20_(S) toward the roller electrodes, and is the largest at the positionaligned with the rotation axis C2 of the roller electrodes, as indicatedby line L in FIG. 4. On the other hand, the retainer rollers 16squeezing the plate-like workpieces 12_(H), 12_(M) generate an inwardfriction force corresponding to the squeezing force of the retainerrollers 16, as a reaction force opposing the expanding force.

The expanding movement of the pair of plate-like workpieces 12_(H),12_(M) away from each other can be prevented when the above-indicatedfriction force is equal to or larger than the expanding force. At theguide rollers 14 disposed on the opposite sides of the guide members18_(U), 18_(S), however, the force component F_(R1) acts on theplate-like workpieces in the direction toward the guide members 18_(U),18_(S), so that the friction force F_(F1) required at the position ofeach guide roller 14 is equal to the expanding force F_(E1) minus thecomponent F_(R1), namely, F_(F1) ≧F_(E1) -F_(R1). The component F_(R1)does not act on a portion of the plate-like workpieces 12_(H), 12_(M) atwhich the workpieces are held by the retainer rollers 16 disposed on theopposite sides of the roller electrodes 20_(U), 20_(S), so that thefriction force F_(F2) required at the position of the retainer rollers16 is equal to or larger than the expanding force F_(E2), namely, F_(F2)/F_(E2).

To prevent the expanding movement of the pair of plate-like workpieces12_(H), 12_(M) away from each other, therefore, the friction forcesF_(F1), F_(F2), that is, the squeezing forces of the guide rollers 14and the retainer rollers 16 are determined so as to satisfy arelationship F_(F1) /F_(E1) --F_(R1) on the opposite sides of the guidemembers 18_(U), 18_(S), and a relationship F_(F2) /F_(E2) on theopposite sides of the roller electrodes 20_(U), 20_(S), respectively. Inthe present embodiment, the rotation axis C2 of each retainer roller 16is determined to be perpendicular to the feeding or welding direction B,so that the guiding direction of the retainer rollers is parallel to thefeeding direction. This arrangement facilitates the control of forcesfor stable adjustment of the predetermined lap amount, since only thefriction force of the retainer rollers 16 should be considered inaddition to the expanding force.

FIG. 5 shows an influence of plate thickness t of the plate-likeworkpieces on the distribution of the expanding force, while FIG. 6shows an influence of the plate thickness t on the maximum expandingforce at the positions of the roller electrodes 20_(U), 20_(S). In thecase of steel plates which are generally used for the manufacture ofautomobiles, the expanding force is zero at a position 150 mm distantfrom the roller electrodes 20_(U), 20_(S) in the upstream direction,when the thickness t is 1.0 mm or smaller, as indicated in FIG. 5.Accordingly, the squeezing forces of the guide rollers 14 at thepositions upstream of the above-indicated position are determined so asto permit the pair of plate-like workpieces 12_(H), 12_(M) to be fed inthe feeding direction B. It is also noted that the maximum expandingforce at the positions of the roller electrodes 20_(U), 20_(S) increasesin linear proportion to the plate thickness as indicated in FIG. 6.Therefore, the required squeezing force of the retainer rollers 16 isautomatically determined by the plate thickness.

The mash seam welding apparatus 10 for practicing the mash seam weldingprocess explained above is constructed as shown in FIGS. 7-9. FIG. 7 isa cross sectional view perpendicular to the feeding direction, showingthe guide rollers 14 and their vicinity of the mash seam weldingapparatus 10. FIG. 8 is a view showing in enlargement the guide rollers14 and their vicinity, while FIG. 9 is a partly cut-away view explainingthe support structure for the roller electrode 20_(U).

Since the guide rollers 14 and the retainer rollers 16 are basicallyidentical in construction to each other, the construction of the guiderollers 14 and the support structure thereof will be described by way ofexample.

Referring to FIGS. 7 and 8, a base frame 50 extends in the weldingdirection B, and a pair of posts 52_(H), 52_(M) are disposed upright onthe base frame 50. On the upper end faces of the posts 52_(H), 52_(M),there are provided the guide rollers 14_(HS), 14_(MS) via base blocks53_(H), 53_(M). A top frame 55 is fixed to the base frame 50 through anouter frame 54. To the top frame 55, there are fixed a pair of left andright cylinder devices 56_(H), 56_(M) for giving the pinching forces tothe pair of upper and lower guide rollers 14_(HU), 14_(HS) and the pairof upper and lower guide rollers 14_(MH), 14_(MS), respectively. Movablerods 58_(H), 58_(M) extend downward from the cylinder devices 56_(H),56_(M), such that the movable rods 58_(H), 58_(S) are movable. To thelower ends of the movable rods 58_(H), 58_(M), there are fixed movableplates 60_(H), 60_(M) on which the guide rollers 14_(HU), 14_(MU).

The set of guide rollers 14_(HS), 14_(MS), 14_(HU), 14_(MU) are providedwith respective integrally formed shafts 66 which are supported byrespective shaft cases 62_(HS), 62_(MS), 62_(HU), 64_(MU) throughbearings 64. The above-indicated shaft cases 62_(HS), 62_(MS) are fixedto the upper end faces of the posts 52_(H), 52_(M), while theabove-indicated shaft cases 62_(HU), 62_(MU) are fixed to the movableplates 60_(H), 60_(M). The guide rollers 14_(HS), 14_(MS), are rotatableabout the axis Cl located below the welding centerline A, while theguide rollers 14_(HU), 14_(MU) are rotatable about the axis C1 locatedabove the welding centerline A. As shown in FIG. 3, the guide members18_(U), 18_(S) are fixed to the cylinder device 56_(H) and the post52_(M), respectively, such that the guide members 18_(U), 18_(S) aredisposed with the predetermined amount of gap left between theirmutually opposed end faces, and extend between the pair of upper andlower guide rollers 14_(HU), 14_(HS) for holding the one plate-likeworkpiece 14_(H), and the pair of upper lower guide rollers 14_(MU),14_(MS) for holding the other plate-like workpiece 12_(M), such that thewelding centerline A is aligned with the widthwise centers of theopposed end faces of the guide members 18_(U), 18_(S).

Each of the shafts 66 of the lower guide rollers 14_(HS), 14_(MS) isconnected to a drive motor 76 through a pair of joints 68, 70, anintermediate shaft 72, and a chain 74. The intermediate shaft 72 isprovided for each of the left and right guide rollers 14_(HS), 14_(MS)of each set of guide rollers 14, and these intermediate shafts 72 areconnected to each other by chains and to the drive motor 76, forsimultaneous rotation at the same speed. In this arrangement, the lowerrollers 14_(HS), 14_(MS) of the guide rollers 16 are rotated by thedrive motor 76 to feed the pair of plate-like workpieces 14_(HS),14_(MS). In the present embodiment, the drive motors 76 function as adriving device for rotating the guide rollers 14. Similar drive motorsare used as a driving device for rotating the retainer rollers 16.

The pairs of upper and lower guide rollers 14_(HU), 14_(HS), 14_(MU),14_(MS) are steel rollers whose outer circumferential surfaces arecovered by an elastic material such as urethane rubber. The pair ofguide rollers 14_(HU), 14_(HS) and the pair of guide rollers 14_(MU),14_(MS) are squeezed toward each other by the respective cylinderdevices 56_(H), 56_(M) which function as a squeezing force applyingdevice, so that the squeezing forces acting on the guide rollers permitthe generation of friction forces enough to generate the component forfeeding the pair of plate-like workpieces 12_(H), 12_(M) and thecomponent for forcing the workpieces toward the guide members 18_(U),18_(S).

The pair of upper and lower retainer rollers 16_(HU), 16_(HS) and thepair of upper and lower retainer rollers 16_(MU), 16_(MS) whichconstitute one set of retainer rollers 16 are constructed similarly tothe guide rollers 14_(HU), 14_(HS), 16_(MU), 14_(MS), as described belowby reference to FIG. 15. The retainer rollers 16 are rotatable about therespective rotation axes C2 described above, and are rotated by thedrive motors 76 at the same peripheral speed as the guide rollers14_(HS), 14_(MS). The pair of upper and lower retainer rollers 16_(HU),16_(HS) and the pair of upper and lower retainer rollers 16_(MU),17_(MS) are steel rollers whose outer circumferential surfaces areknurled to increase the generated friction forces in their axialdirection, with respect to the plate-like workpieces 12_(H), 12_(M). Theretainer rollers 16_(HS), 16_(HU) and the retainer rollers 16_(MS),16_(MU) are squeezed toward each other by cylinder devices similar tothe cylinder devices 56_(H), 56_(M), which also function as a squeezingforce applying device. The squeezing forces given to the retainerrollers 16 are comparatively large permitting the generation of frictionforces on their outer circumferential surfaces, so as to prevent themovements of the edge portions of the pair of plate-like workpieces12_(H), 12_(M) under welding in the directions away from each other.

The pair of roller electrodes 20_(U), 20_(S) have the same construction,as shown in FIG. 15, and only the upper roller electrode 20_(U) will beexplained by reference to FIG. 9, which is a partly cut-away viewshowing the upper roller electrode 20_(U) and the support structure ofthis electrode. As shown in FIG. 9, a cylinder device 80_(U) is fixed tothe above-described top frame 55, for giving the roller electrodes20_(U), 20_(S) a squeezing force. The cylinder device 80_(U) is providedwith movable rods 82_(U) extending downward such that the movable rods82_(U) are movable. A movable plate 84 is fixed to the ends of themovable rods 82_(U), and a support block 86 for rotatably supporting theroller electrode 20_(U) is fixed to the movable plate 84.

The roller electrode 20_(U) comprised of a rotary shaft 90_(U) with aflange portion 88_(U) integrally formed at an axially intermediate partthereof, and a circular electrode disk 96_(U) fastened by a screw 94_(U)to the flange portion 88_(U) together with a pressure plate 92_(U). Therotary shaft 90_(U) is fitted at its opposite end portions through powersupply bushings 98_(U), in a through-hole 100_(U) formed through theabove-indicated support block 86U, whereby the rotary shaft 90_(U) isrotatably supported by the support block 86_(U). The above electrodedisk 96_(U) is a highly electrically conductive body formed of ametallic material such as chromium steel, beryllium copper alloy orchromium zirconium copper alloy, which is highly durable with a smallamount of wear or damage even in the presence of a comparatively largeelectric current applied thereto while the metallic material is incontact with the plate-like workpieces 12_(H), 12_(M). The rotary shaft90_(U), pressure plate 92_(U), power supply bushings 98_(U) and supportblocks 86_(U) are highly electrically conductive bodies formed of ametallic material such as copper alloy or aluminum alloy. The electriccurrent is supplied from the above-indicated power source 22 to theelectrode disk 96_(U) through electric wires not shown, and through thepower supply bushings 98, rotary shaft 90_(U), etc.

A fluid passage 102_(U) is formed through the above-indicated rotaryshaft 90_(U) such that the fluid passage 102_(U) is open in the oppositeend faces of the rotary shaft 90_(U) and has an intermediate portion inthe form of a circular recess formed in the surface of the flangeportion 88_(U) which contacts the electrode disk 96_(U). A pair of hosejoints 108_(U), 110_(U) are fixed to the end faces of the rotary shaft90_(U), for connecting the fluid passage 102_(U) to coolant fluid hoses104_(U), 106_(U) while permitting the rotary shaft 90_(U) to rotate.

In the mash seam welding apparatus 10 constructed as described above,the plurality of sets of guide rollers 14 and the one set of retainerrollers 16 are continuously rotated by the drive motors 76, so that thepair of plate-like workpieces 12_(H), 12_(M) are fed and positionedrelative to each other by the guide rollers 14 and the guide members18_(U), 18_(S) such that the edge portions of the workpieces overlapeach other over the predetermined lap width S. Then, the workpieces aresqueezed and mashed by the pair of roller electrodes 20_(U), 20_(S), anda welding current is simultaneously applied between the rollerelectrodes 20_(U), 20_(S), whereby the workpieces are welded at theiredge portions as shown in FIG. 13, for example, as described below.Therefore, the present embodiment does not require a mechanism asprovided in the apparatus shown in FIG. 10, for instance, for returningthe upper and lower clamping members 120 after removal thereof from thewelded plate-like workpieces 12_(H), 12_(M). The apparatus of FIG. 10uses the clamping mechanism 122 for moving the pair of plate-likeworkpieces 12_(H), 12_(M) to the pair of roller electrodes 20_(U),20_(S) while the workpieces are held by the upper and lower claimingmembers 120 such that the edge portions of the workpieces overlap eachother. Accordingly, the seam welding apparatus according to the presentembodiment is less complicated and smaller is size than the apparatus ofFIG. 10. Further, the present embodiment is free from deterioration ofseam welding efficiency due to movements of the roller electrodes20_(U), 20_(S), and permits higher seam welding efficiency than in anapparatus adapted to reciprocate the roller electrodes 20_(U), 20_(S)relative to the upper and lower clamping members 120.

Further, the present embodiment uses the guide members 18_(U), 18_(S)which are fixedly disposed upstream of the roller electrodes 20_(U),20_(S) and which have the pair of guide surfaces 24_(U), 24_(S) parallelto the direction perpendicular to the axial direction of the pair ofroller electrodes 20_(U), 20_(S). When the two plate-like workpieces12_(H), 12_(M) are fed, these workpieces are positioned relative to eachother such that the end faces of the overlapping edge portions are insliding contact with the respective guide surfaces 24_(U), 24_(S). Thus,the present embodiment assures improved positioning accuracy of theoverlapping edge portions of the two plate-like workpieces 12_(H),12_(M), with a simple structural arrangement.

In the present embodiment, at least two pairs of guide rollers 14_(HU)and 14_(HS), 14_(MU) and 14_(MS) are provided on the opposite sides ofthe guide members 18_(U), 18_(S), for pinching the two plate-likeworkpieces 12_(H), 12_(M) in the direction of thickness of theworkpieces, and for guiding the two plate-like workpieces 12_(H), 12_(M)such that the workpieces are moved toward the guide surfaces 24_(U),24_(S) of the guide members 18_(U), 18_(S) as the workpieces are fedtoward the roller electrodes 20_(U), 20_(S), so that the end faces ofthe edge portions of the two plate-like workpieces 12_(H), 12_(M) areheld in sliding contact with the guide surfaces 24_(U), 24_(S). Thus,the two plate-like workpieces 12_(H), 12_(M) are forced onto the guidesurfaces 24_(U), 24_(S) of the guide members 18_(U), 18_(S) as theworkpieces are fed by the two pairs of guide rollers 14_(HU), 14_(HS),14_(MU), 14_(MS). This arrangement is advantageous for a reduced numberof sliding portions, than in a guiding arrangement which uses inclinedguide plates for forcing the plate-like workpieces 12_(H), 12_(M) ontothe guide surfaces 24_(U), 24_(S) of the guide members 18_(U), 18_(S).

The present embodiment is adapted to weld the two plate-like workpieces12_(H), 12_(M) at their edge portions while the edge portions aresqueezed to be mashed by and between the pair of roller electrodes20_(U), 20_(S). Accordingly, it is not necessary to chamfer the edgeportions of the plate-like workpieces 12_(H), 12_(M) prior to thewelding operation. It is also noted that the conventional apparatusadapted to perform a so-called mash seam welding process requires alarge clamping mechanism for resisting a large expanding force generatedby mashing of the two plate-like workpieces 12_(H), 12_(M) by the pairof roller electrodes 20_(U), 20_(S). However, the present apparatus isadvantageous for reduced size thereof.

That is, the present embodiment is provided with the two pairs ofretainer rollers 16_(HU) and 16_(HS), and 16_(MU) and 16_(MS) which aredisposed on the opposite sides of the pair of roller electrodes 20_(U),20_(S), and a squeezing force applying device (cylinder device) forapplying the squeezing forces to these two pairs of retainer rollers16_(HU) and 16_(HS), and 16_(MU) and 16_(MS). The two pairs of retainerrollers 16_(HU), 16_(HS), 16_(MU), 16_(MS) are disposed rotatably abouttheir axes aligned with the axes of the pair of roller electrodes20_(U), 20_(S), for squeezing the two plate-like workpieces 12_(H),12_(M) in their direction of thickness. The squeezing forces given bythe squeezing force applying device are enough to permit the retainerrollers to generate a friction force sufficient to resist the expandingforce of the two plate-like workpieces 12_(H), 12_(M) generated in thewelding process by the pair of roller electrodes 20_(U), 20_(S). Thisarrangement prevents expanding movements of the two plate-likeworkpieces 12_(H), 12_(M) away from each other during welding at theiredge portions, and is therefore effective to improve the accuracy of thewelding position.

There will next be described other embodiments of this invention. In thefollowing embodiments, the reference numerals used in the precedingembodiment will be used to identify the corresponding elements, and nodescription of these elements will be provided.

In the preceding embodiment, the plate-like workpieces 12_(H), 12_(M)may undergo buckling or convex deformation F due to an expanding forcegenerated in the axial direction of the roller electrodes 20_(U),20_(S), between the roller electrodes 20_(U), 20_(S) and the two pairsof retainer rollers 16_(HU) and 16_(HS), and 16_(MU) and 16_(MS), asindicated in FIG. 11. Such convex deformation F is more likely to takeplace as the thickness of the plate-like workpieces 12_(H), 12_(M)becomes thinner. The convex deformation F tends to be less likely totake place when the two pairs of retainer rollers 16_(HU), 16_(HS),16_(MU), 16_(MS) are located closer to the roller electrodes 20_(U),20_(S), as shown in FIG. 12. In this respect, it is desirable to disposethe two pairs of retainer rollers 16_(HU), 16_(HS), 16_(MU), 16_(MS) asclose as possible to the roller electrodes 20_(U), 20_(S). When theabove convex deformation F does not take place, the edge portions of theplate-like workpieces 12_(H), 12_(M) are suitably mashed such that aninterface G of the edge portions is inclined by a relatively smallangle, so that a weld nugget N is.formed due to the welding current, asindicated in FIG. 13, at an intermediate position in the direction ofthickness of the workpieces, and near the interface G at which theelectrical resistance is high. When the convex deformation F takesplace, on the other hand, the edge portions of the plate-like workpieces12_(H), 12_(M) are displaced in the right and left directions, with aresult of an increase in the angle of inclination of the interface G,and a consequent increase in the area of contact of the electrode discs96_(U), 96_(S) with the non-overlapping portions of the plate-likeworkpieces 12_(H), 12_(M), as indicated in FIG. 14, whereby the weldnugget is unlikely to be formed, making it difficult to achieve the seamwelding. Broken lines with arrows in FIGS. 13 and 14 indicate thewelding current.

However, there is an upper limit D in the distance between the retainerrollers and the roller electrodes 20_(U), 20_(S), because the supportblocks 86 for supporting the roller electrodes 20_(U), 20_(S) and theother members are located near the roller electrodes, and because thesupport blocks 86_(U), 86_(S) come closer to the plate-like workpieces12_(H), 12_(M) as the diameters of the roller electrodes 20_(U), 20_(S)decrease due to wear, as indicated by broken lines in FIG. 15.Therefore, it may be impossible to prevent the above-indicated convexdeformation F of the plate-like workpieces 12_(H), 12_(M), dependingupon the thickness of the workpieces.

FIG. 15 shows an embodiment in which a presser device is provided forpreventing the occurrence of the above-indicated convex deformation F.In the apparatus of FIG. 15, auxiliary roller sets 130_(HU), 130_(HS),130_(MU), 130_(MS) are interposed between the roller electrodes 20_(U),20_(S) and the pair of retainer rollers 16_(HU), 16_(HS), and betweenthe roller electrodes 20_(U), 20_(S) and the pair of retainer rollers16_(MU), 16_(MS), in the direction perpendicular to the feedingdirection B, that is, in the axial direction of the roller electrodes.These auxiliary roller sets 130_(HU), 130_(HS), 130_(MU), 130_(MS) arelocated so as to press the portions of the workpieces at which theconvex deformation F is expected to take place. Described morespecifically, the auxiliary roller sets 130_(HS) and 130_(MS) locatedbelow the plate-like workpieces 12_(H), 12_(M) are rotatably supportedby the end portions of brackets 132_(HS) and 132_(MS) fixed to the baseblocks 53_(H), 53_(M), while the auxiliary roller sets 130_(HU) and130_(MU) located above the plate-like workpieces 12_(H), 12_(M) arerotatably supported by the end portions of brackets 132_(HU) and132_(MU) fixed to the movable plates 60_(H), 60_(M). The above auxiliaryroller sets 130_(HU), 130_(HS), 130_(MU), 130_(MS) are rotatable aboutaxes C4 which are parallel to the rotation axes C3 of the rollerelectrodes 20_(U), 20_(S) and the axes C2 of the pair of retainerrollers 16_(HU), 16_(HS). The plate-like workpieces 12_(H), 12_(M) arepressed with predetermined squeezing forces at their portions laterallyadjacent to the roller electrodes 20_(U), 20_(M), by the upper and lowerauxiliary roller sets 130_(HU), 130_(HS) and the upper and lowerauxiliary roller sets 130_(MU), 130_(MS).

As also shown in FIG. 16, each of the above auxiliary roller sets130_(HU), 130_(HS), 130_(MU), 130_(MS) consists of three rollers whichare arranged in the feeding direction B on the end portion of theappropriate bracket 132_(HU), 132_(MU) 132_(HS), 132_(MS), so as toapply sufficient pressing forces to prevent the convex deformation Fwhich would otherwise take place on the plate-like workpieces 12_(H),12_(M) in the feeding direction B. To permit the above auxiliary rollers130_(HU), 30_(HS), 130_(MU), 130_(MS) to be located sufficiently closeto the roller electrodes 20_(U), 20_(S), the diameter of the auxiliaryrollers is made smaller than the distance between the plate-likeworkpiece 12_(H), 12_(M) and the end of the appropriate support block86_(U), 86_(S) as positioned (the position being indicated by brokenlines in FIG. 15) when the roller electrode 20_(U), 20_(S) has thelargest amount of wear, so that the auxiliary rollers may be locatedbetween the ends of the support blocks 86_(U), 86_(S) and the plate-likeworkpieces 12_(H), 12_(M). FIG. 16 is a view showing the brackets132_(HU), 132_(HS) on the left side of the feeding direction B, and theauxiliary rollers 130_(HU), 130_(HS) rotatably supported by thesebrackets 132_(HU), 132_(HS), which view is taken from the rollerelectrodes 20_(U), 20_(S) toward the auxiliary rollers. In the presentembodiment, the above-indicated auxiliary rollers 130_(HU), 130_(HS),130_(MU), 130_(MS) and the brackets 132_(HU), 132_(HS), 132_(MU),132_(MS) supporting these auxiliary rollers function as the presserdevice for preventing the convex deformation F.

In the present embodiment, the auxiliary rollers 130_(HU), 130_(HS),130_(MU), 130_(MS) and the brackets 132_(HU), 132_(HS), 132_(MU),132_(MS) supporting these auxiliary rollers are provided to function asthe presser device for preventing the convex deformation F of theplate-like workpieces 12_(H), 12_(M) between the roller electrodes20_(U), 20_(S) and the retainer rollers 16_(HU), 16_(HS) and between theroller electrodes 20_(U), 20_(S) and the retainer rollers 16_(MU),16_(MS). The presser device is adapted to press (perform a pressing stepon) the portions of the plate-like workpieces 12_(H), 12_(M) at whichthe convex deformation F is expected to take place, whereby the convexdeformation F indicated by one-dot chain lines in FIG. 17 is prevented,even if the plate-like workpieces 12_(H), 12_(M) have small thicknessvalues. Accordingly, the presser device can suitably prevent thedisplacement of the edge portions of the plate-like workpieces 12_(H),12_(M) away from each other due to the convex deformation F, and theresulting failure of formation of the weld nugget N within the thicknessof the plate-like workpieces 12_(H), 12_(M) due to the abovedisplacement.

In an embodiment shown in FIG. 18, auxiliary skids 134_(HU), 134_(HS),134_(MU), 134_(MS) for sliding contact with the plate-like workpieces12_(H),. 12_(M) are provided on the end portions of the brackets132_(HU), 132_(HS), 132_(MU), 132_(MS), in place of the auxiliaryrollers 130_(HU), 130_(HS), 130_(MU), 130_(MS) provided in the aboveembodiment of FIG. 15. These auxiliary skids 134_(HU), 134_(HS),134_(MU), 134_(MS) have a height dimension which is smaller than thedistance between the plate-like workpiece 12_(H), 12_(M) and the end ofthe support block 86_(U), 86_(S) as positioned (the position beingindicated by broken lines) when the roller electrode 20_(U), 20_(S) hasthe largest amount of wear, so that the auxiliary skids may be locatedbetween the ends of the support blocks 86_(U), 86_(S) and the plate-likeworkpieces 12_(H), 12_(M). As also shown in FIG. 19, the auxiliary skids134_(HU), 134_(HS), 134_(MU), 134_(MS) are adapted to press theplate-like workpieces 12_(H), 12_(M) on their upper and lower surfaces,at the same positions as the above auxiliary rollers 130_(HU), 130_(HS),130_(MU), 130_(MS). Preferably, the auxiliary skids 134_(HU), 134_(HS),134_(MU), 134_(MS) are formed of a material such as sintered metalimpregnated with molybdenum disulfide or carbon, which has a high wearresistance and a low friction coefficient. The present embodiment hasthe same advantage as the embodiment of FIG. 15. It is noted that FIG.19 is a view showing the right auxiliary skids 134_(MU), 134_(MS) andthe roller electrodes 20_(U), 20_(S), which view is taken from the rightside of the feeding direction.

FIG. 20, which illustrates another embodiment of this invention, showsthe roller electrode 20U rotated by an electric motor 140. In theembodiment of FIG. 20, the pressure plate 92_(U) for holding thecircular electrode disk 96_(U) under pressure onto the flange portion88_(U) of the rotary shaft 90_(U) is formed with outer circumferentialteeth 142 which meshes with an intermediate gear 144 rotatably supportedby the support block 86_(U). The electric motor 140 is provided with anoutput gear 146 which engages the pressure plate 92_(U) through theintermediate gear 144, and is fixed to the support block 86_(U). Theelectric motor 140 is adapted to rotate the roller electrode 20_(U) sothat the peripheral speed of the roller electrode 20_(U) when the rollerelectrode 20_(U) is not in contact with the plate-like workpiece 12_(H)is higher than the feeding speed of the workpiece 12_(H), whereby theroller electrode 20_(U) transfers a driving force to the plate-likeworkpiece 12_(H) to feed it in the feeding direction after the workpiece12_(H) is brought into contact with the roller electrode 20_(U). It isnoted that FIG. 20 is a view developed about the axis of theintermediate gear 144, regarding the positional relationship between theelectric motor 140, intermediate gear 144 and their vicinity.

When the edge portions of the pair of plate-like workpieces 12_(H),12_(M) are squeezed by the roller electrodes 20_(U), 20_(S) in the seamwelding operation, the expanding force acts on the plate-like workpieces12_(H), 12_(M) in a radial direction from the point of squeezing by theroller electrodes 20_(U), 20_(S), as indicated by arrows in FIG. 21. Theradial components of the squeezing force which act in the rear(upstream) direction toward the unwelded portions (in the weldingdirection from the welded portion toward the weld end) are considered tohave the largest influence on the welding process. In the presentembodiment, the roller electrode 20_(U) is rotated by the electric motor140 to transfer the driving force to the plate-like workpiece 12_(H) inthe feeding direction during the seam welding (Roller Electrode DrivingStep). Thus, the roller electrode 20_(U) produces a drive force F_(D) inthe feeding direction, as indicted in FIGS. 21 and 22. This drive forceF_(D) cooperates with a radial component of the squeezing force actingin the direction toward the unwelded portion, for example, a componentF_(p), to constitute a resultant force F_(T) as indicated in FIG. 22,which resultant force F_(T) includes a component acting in the directiontoward the welded portion. However, the welded portion of the plate-likeworkpieces 12_(H), 12_(M) is not significantly influenced by the aboveresultant force F_(T). Accordingly, the radial components of thesqueezing force which act in the welding direction from the weldedportion toward the weld end are restricted, whereby the generation ofbuckling or convex deformation F of the plate-like workpieces 12_(H),12_(M) is suitably eliminated. Thus, the present embodiment is adaptedto restrict the expanding force of the plate-like workpieces 12_(H),12_(M), and is advantageously effective to suitably prevent theformation of the weld nugget at a position apart from the interface theedge portions of the plate-like workpieces 12_(H), 12_(M) due to thedisplacement of the edge portions away from each other while beingsqueezed between the roller electrodes 20_(U), 20_(S).

FIG. 23 schematically shows a portion of a sea welding apparatus ofanother embodiment of this invention. In the apparatus of FIG. 23, thereis provided a coolant supply device 150 comprised of a coolant supplysource 152 and an injection nozzle 154. The coolant supply source 152 isprovided with a pump or accumulator for delivering a coolant fluid suchas air, gas, water or oil under pressure, and a cooling device as neededto cool the coolant fluid. The coolant fluid under pressure is deliveredto the injection nozzle 154. During the welding operation, the coolantfluid delivered from the above coolant supply source 152 is injectedfrom the injection nozzle 154 to the point of squeezing of theworkpieces by the roller electrodes 20_(U), 20_(S) and its vicinity,that is, to the portion of the workpieces which is heated by applicationof the electric current by the roller electrodes 20_(U), 20_(S), wherebythe plate-like workpieces 12_(H), 12_(M) are cooled (Coolant SupplyStep). In this embodiment, the portion of the plate-like workpieces12_(H), 12_(M) which is heated by the application of the electriccurrent by the roller electrodes 20_(U), 20_(S) in the seam weldingoperation is cooled by the coolant supply device 150, so as to restrictthe expanding force of the plate-like workpieces 12_(H), 12_(M) due toheat of the weld nugget N formed within the workpieces 12_(H), 12_(M)between the roller electrodes 20_(U), 20_(S), whereby the difficulty informing the weld nugget N within the workpieces 12_(H), 12_(M) issuitably prevented.

FIG. 24 schematically shows a portion of a seam welding apparatus ofanother embodiment of this invention. This embodiment is different fromthe embodiment of FIG. 1 mainly in that four sets of retainer rollers 16are provided in the present embodiment while only one set of retainerroller 16 is provided in the embodiment of FIG. 1. IN the apparatus ofFIG. 14, the pair of plate-like workpieces 12_(H), 12_(M) introduced ata predetermined position by a work loading device (not shown) such as apusher device are fed in the feeding direction B (Feeding Step), whilethe workpieces are guided in sliding contact with the guide surfaces24_(U), 24_(S) of the pair of upper and lower guide members 18_(U),18_(S) (Guiding Step), such that the overlapping edge portions of theworkpieces are positioned relative to each other so as to establish thepredetermined lap amount (lap width) S. Then, the pair of plate-likeworkpieces 12_(H), 12_(M) are seam-welded together (Welding Step) with awelding current applied between the pair of roller electrodes 20_(U),20_(S) while the workpieces are squeezed by the pair of upper and lowerroller electrodes 20_(U), 20_(S) at their overlapping edge portionspositioned relative to each other. The seam welding of the pair ofplate-like workpieces 12_(H), 12_(M) is effected along the weldingcenterline A which is parallel to the feeding direction B and whichpasses the center of the lap width S of the overlapping edge portions ofthe workpieces. In the welding step, the pair of plate-like workpieces12_(H), 12_(M) are squeezed in the direction of thickness with apredetermined squeezing force by the four sets of retainer rollers 16disposed on the opposite sides of the roller electrodes 20_(U), 20_(S),whereby the plate-like workpieces 12_(H), 12_(M) are prevented frombeing moved away from each other (Holding Step). The pairs of plate-likeworkpieces 12_(H), 12_(M) which have been welded together are unloadedby a conveyor 160 and placed in stack on a pallet 162.

While the embodiments of this invention have been explained referring tothe drawings, the invention may be otherwise embodied.

For example, the illustrated embodiments adapted to seam-weld theplate-like workpieces 12_(H), 12_(M) at their edge portions may bemodified to seam-weld the workpieces at their portions located inwardlyfrom the edge portions.

Further, although the illustrated embodiments are adapted to effect aso-called mash seam welding operation in which the edge portions of theplate-like workpieces 12_(H), 12_(M) are squeezed and mashed by the pairof roller electrodes 20_(U), 20_(S), the invention is applicable to aseam welding operation in which the workpieces are merely squeezed bythe roller electrodes 20_(U), 20_(S). In this case, the edge portions ofthe plate-like workpieces 12_(H), 12_(M) which are to contact each othermay be subjected to a preliminary treatment such as a chamberingoperation to remove parts of the edge portions or a mashing operation tomash and shape the edge portions.

In the illustrated embodiments, the guide rollers 14 and the retainerrollers 16 include the lower guide rollers 14_(HS), 14_(MS) and thelower retainer rollers 16_(HS), 16_(MS) which are located below theplate-like workpieces 12_(H), 12_(M) and which are driven by the drivemotors 76. However, the upper guide rollers 14_(HU), 14_(MU) and theupper retainer rollers 16_(HU), 16_(MU) are driven by the drive motors76, or the upper and lower guide and retainer rollers are both driven bythe drive motors 76.

While all of the guide rollers 14_(HS), 14_(MS) are driven by the drivemotors 76 in the illustrated embodiments, only the selected ones ofthese guide rollers may be driven. In the case where the pair ofplate-like workpieces 12_(H), 12_(M) can be sufficiently fed by aplurality of sets of guide rollers 14, the set of retainer rollers 16 inthe embodiments of FIGS. 1-9 need not be driven.

Further, at least one of the pair of roller electrodes 20_(U), 20_(S) inthe embodiment of FIGS. 1-9 may be driven by the electric motor 140shown in FIG. 20, for example, at a peripheral speed equal to thefeeding speed of the pair of plate-like workpieces 12_(H), 12_(M).

In the embodiment of FIGS. 1-9, the guide rollers 14 are inclined by apredetermined angle θ for pressing the pair of plate-like workpieces12_(H), 12_(M) toward the guide members 18_(U), 18_(S) while theworkpieces are fed. However, slant guiding plates may be provided forcontact with the outer edges of the pair of plate-like workpieces12_(H), 12_(M), such that the slant guiding plates approach the guidemembers 18_(U), 18_(S) as it extends in the feeding direction.

It is to be understood that the foregoing embodiments are provided forillustrative purpose only, and that various changes may be made in theillustrated embodiments, without departing from the spirit of theinvention.

We claim:
 1. A seam welding process of continuously welding twoplate-like workpieces at overlapping edge portions thereof while saidoverlapping edge portions are squeezed by a pair of roller electrodesdisposed rotatably about respective two parallel axes thereof, said seamwelding process comprising:a feeding step of feeding said two plate-likeworkpieces in a feeding direction toward said roller electrodes by aplurality of sets of guide rollers such that said two plate-likeworkpieces are pinched by said guide rollers in a direction of thicknessthereof while said guide rollers are rotated; a guiding step of guidingsaid two plate-like workpieces being fed by said guide rollers, suchthat said edge portions overlap each other with a predetermined lapamount so that the overlapping edge portions are squeezed by said pairof roller electrodes; and a welding step of continuously welding saidedge portions of said two plate-like workpieces squeezed by said pair ofroller electrodes, by applying a welding current between said pair ofroller electrodes.
 2. A seam welding process according to claim 1,wherein said guiding step comprises providing a guide member which isfixedly disposed upstream of said pair of roller electrodes in saidfeeding direction (B) and which has a pair of guide surfaces extendingin a direction perpendicular to said axes of said roller electrodes, andguiding said two plate-like workpieces such that end faces of saidoverlapping edge portions of said two plate-like workpieces are insliding contact with said pair of guide surfaces.
 3. A seam weldingprocess according to claim 2, wherein said plurality of sets of guiderollers include at least two pairs of guide rollers, and said guidingstep comprises locating said at least two pairs of guide rollers onopposite sides of said guide member to pinch said two plate-likeworkpieces in the direction of thickness, and operating said at leasttwo pairs of guide rollers so as to generate forces including componentswhich acting in directions toward said pair of guide surfaces of saidguide member while said two plate-like workpieces are fed, whereby saidend faces of said overlapping edge portions of said two plate-likeworkpieces are brought into sliding contact with said guide surfaces. 4.A seam welding process according to claim 1, wherein said weldingprocess comprises welding said two plate-like workpieces while saidoverlapping edge portions are squeezed to be mashed by and between saidpair of roller electrodes which are biased toward each other.
 5. A seamwelding process according to claim 1, further comprising a holding stepof squeezing, in said welding step, said two plate-like workpieces inthe direction of thickness with a predetermined squeezing force by twopairs of retainer rollers disposed on opposite sides of said pair ofroller electrodes, respectively, such that said two pairs of retainerrollers generate friction forces enough to resist expanding forces ofsaid two plate-like workpieces in said welding step, to thereby preventmovements of said two plate-like workpieces away from each other.
 6. Aseam welding process according to claim 5, further comprising a pressingstep of pressing said plate-like workpieces at portions thereof whichare located between said roller electrodes and said retainer rollers inan axial direction of said roller electrodes and at which convexdeformation is expected to take place in said welding step.
 7. A seamwelding process according to claim 1, further comprising an electrodedriving step of rotating said roller electrodes so as to transfer adriving force to said plate-like workpieces in said feeding direction.8. A seam welding process according to claim 1, further comprising astep of supplying a coolant fluid to portions of said plate-likeworkpieces (12) which are squeezed by said roller electrodes (20) andheated in said welding step by said roller electrodes.
 9. A seam weldingapparatus for continuously welding two plate-like workpieces atoverlapping edge portions thereof while said overlapping edge portionsare squeezed by a pair of roller electrodes disposed rotatably aboutrespective two parallel axes thereof and while a welding current isapplied between said pair of roller electrodes, said seam weldingapparatus comprising:a pair of roller electrodes disposed rotatablyabout respective two parallel axes thereof and biased toward each other;a plurality of sets of guide rollers which are disposed on oppositesides of said two plate-like workpieces in a direction of thickness ofsaid workpieces and which are rotatable about axes thereof substantiallyparallel to said axes of said roller electrodes; a driving device forrotating at least one set of said plurality of sets of guide rollers ina predetermined rotating direction, to feed said two plate-likeworkpieces in a feeding direction while said two plate-like workpiecesare pinched by said guide rollers; and a guiding device for guiding saidtwo plate-like workpieces being fed in said feeding direction by saiddriving device, such that said edge portions overlap each other with apredetermined lap amount so that the overlapping edge portions aresqueezed by said pair of roller electrodes.
 10. A seam welding apparatusaccording to claim 9, wherein said guide device includes a guide memberwhich is fixedly disposed upstream of said pair of roller electrodes andwhich has a pair of guide surfaces extending in a directionperpendicular to said axes of said roller electrodes, said guide devicepositioning said two plate-like workpieces relative to each other duringfeeding of said plate-like workpieces such that end faces of saidoverlapping edge portions of said two plate-like workpieces are insliding contact with said pair of guide surfaces.
 11. A seam weldingapparatus according to claim 10, wherein said guide device furtherincludes at least two pairs of guide rollers located on opposite sidesof said guide member to pinch said two plate-like workpieces in thedirection of thickness, said at least two pairs of guide rollers guidingsaid two plate-like workpieces such that said two plate-like workpiecesapproach said pair of guiding surfaces of said guide member as said twoplate-like workpieces approach said roller electrodes, whereby said endfaces of said overlapping edge portions of said two plate-likeworkpieces are brought into sliding contact with said guide surfaces.12. A seam welding apparatus according to claim 9, wherein said pair ofroller electrodes weld said edge portions of said two plate-likeworkpieces while said edge portions of the plate-like workpieces aresqueezed to be mashed by said pair of roller electrodes.
 13. A seamwelding apparatus according to claim 9, further comprising:two pairs ofretainer rollers which are disposed on opposite sides of said pair ofroller electrodes and which are rotatable about respective axes parallelto said axes of said pair of roller electrodes, said two pairs ofretainer rollers squeezing said two plate-like workpieces in thedirection of thickness while said two plate-like workpieces are squeezedby said pair of roller electrodes; and a squeezing force applying devicefor applying to said two pairs of retainer rollers squeezing forceswhich permit said retainer rollers to generate friction forces enough toresist expanding forces of said two plate-like workpieces which aregenerated during welding thereof by said pair of roller electrodes. 14.A seam welding apparatus according to claim 13, further comprising apresser device (130, 134) for pressing portions of said plate-likeworkpieces (12) which are located between said roller electrodes (20)and said retainer rollers (16) in an axial direction of said rollerelectrodes and at which convex deformation is expected to take place.15. A seam welding apparatus according to claim 14, wherein said presserdevice includes auxiliary rollers which are interposed between said pairof roller electrodes and said retainer rollers in said axial directionof said roller electrodes and which are rotatable about respective axesparallel to said axes of said roller electrodes, said auxiliary rollerspressing said two plate-like workpieces while permitting said plate-likeworkpieces to be fed in said feeding direction.
 16. A seam weldingapparatus according to claim 14, wherein said presser device includesauxiliary skids which are interposed between said roller electrodes andsaid retainer rollers in said axial direction of said roller electrodes,for sliding contact with said plate-like workpieces.
 17. A seam weldingapparatus according to claim 9, further comprising an electrode drivingdevice for rotating said roller electrodes so as to transfer a drivingforce to said plate-like workpieces in the feeding direction.
 18. A seamwelding apparatus according to claim 9, further comprising a coolantsupply device for supplying a coolant fluid to portions of saidplate-like workpieces which are squeezed and heated for seam welding bysaid roller electrodes.